1. Field of the Invention.
The present invention relates to a process for the preparation of high purity silane suitable for forming thin semiconductor and dielectric layers, and also high purity poly- and single crystal silicon, for a variety of applications, such as electronics and solar energy.
2. Description of the Prior Art
Thermal decomposition of silane is the most favorable of all known techniques for high purity silicon preparation even when compared to the currently most common method of hydrogen reduction of trichlorosilane. Arguments in favor of the thermal decomposition technique include: (1) thermal decomposition proceeds at a temperature of about 850.degree. C., instead of 1100.degree. C., and is, therefore, less energy consuming; (2) products of the reaction do not contain hydrogen chloride, chlorosilanes, and other reactive compounds which reduce the purity of the silicon produced; (3) significant differences in physical and chemical properties of silane and impurity compounds allow for a more effective separation of silane from the majority of undesirable impurities; and (4) not only silicon, but silane, and its gaseous mixtures, are also industrial products that are needed for the production of thin-film semiconductor devices. The majority of the processes for silicon preparation from silane are complicated and more expensive than the trichlorosilane reduction method. For that reason, silane is used only for highest purity polysilicon that is transformed into single crystal silicon by float-zone melting. This single crystal silicon is used in infrared receivers and nuclear radiation detectors. In order to reduce the cost of silane and silicon, while maintaining high purity of the materials, scientists around the world are searching for new and more effective technologies for production of high-purity monosilane. A well-known technique for silane preparation is simultaneous reduction and oxidation of trichlorosilane (German Federal Republic Patent No. 3,311,650, published on Oct. 13, 1983). The first step of the process is catalytic hydrogenation of metallurgical silicon: ##STR1## The process proceeds at 400-600.degree. C. in the pressure range from 0.7 to 41.4 bar. It is followed by di- and trichlorosilane extraction. The second step is the simultaneous reduction and oxidation of chIorosilanes in the presence of a catalyst. The resulting reaction is: ##STR2##
Anion-exchange tars with ternary amines are used as a catalyst. The reduction-oxidation reaction is carried out at temperatures up to 150.degree. C. It is followed by silane purification. In this method, secondary hydrogenation is carried out on silicon tetrachloride, that is created as a byproduct when monosilane is obtained by simultaneous oxidation and reduction of chlorosilanes, which allows high efficiency in the consumption of raw metallurgical silicon and significantly reduces the cost of monosilane and polycrystalline silicon. This method, however, has several significant disadvantages. First, it requires special equipment designed to withstand the high temperature, and high pressures of the silicon tetrachloride hydrogenation process. Second, it has aggressive chlorine compounds that carry impurities from the walls of the reactor into monosilane. Third, difficulties associated with the purification of silane, up to high-purity grade, are encountered. Finally, ecologically unsafe chlorine compounds are involved in the process.
The basics of simultaneous catalytic oxidation and reduction of alcoxysilane, in particular of the triethoxysilane, were developed in the USSR, in 1957-1959 (Soviet Journal of Technical Physics, 1957, v. 27, No. 8, pp. 1645-1648, Soviet Journal of Solid State Physics, 1959, v. 1, pp. 999-1001). Further development of this method, especially in obtaining alcoxysilane by direct reaction between metallurgic silicon and ethanol, is described in Japanese Patent No. 7,427,517, dated Jul. 18, 1974. We consider this patent to be a precursor of the current invention, and it is incorporated by reference herein.
According to this precursor technology, high purity silane synthesis starts with the reaction of metallurgical silicon with ethanol in the presence of a catalyst at elevated temperatures. Triethoxysilane, separated from the products of the reaction and purified, undergoes the simultaneous oxidation and reduction reaction in the presence of a catalyst. Silane formed in this reaction is separated from other products and is purified from the mixture by low temperature condensation and sorption on different adsorbents. This process is conducted in the following manner. The reaction of powdered silicon with ethanol proceeds at 190-200.degree. C. The catalyst, in this reaction, is an element of Ib group of the Periodic Table (for example Cu or Ag). The medium, of the reaction, is a solvent with a high boiling temperature, such as hexaisopropoxysiloxane, with a boiling temperature of 280.degree. C., or dodecaethoxypentasiloxane with boiling temperature 290-300.degree. C. The reaction of silicon with ethanol proceeds according to the equation: ##STR3##
Ideally n equals 1 but it is usually less than 1. The reaction shifts toward one or another product depending on properties of the catalyst and the medium.
Triethoxysilane extracted from the products, of the reaction, is purified by multistage distillation. Then, it is subjected to the simultaneous reduction and oxidation reaction in the presence of a catalyst. The catalyst in this case comprises elements of groups Ia and IIa of the Periodic Table (Na, K, Ca, and etc.). Low temperature condensation and adsorption techniques are used to purify silane formed in the simultaneous oxidation and reduction reaction. The most effective adsorbents appear to be activated carbon, activated aluminosilicate, and silica gel.
Important problems related to purity of the produced silane and efficiency of the process had not been addressed in the precursor method, Japanese Patent No. 7,427,517. Solving these problems would allow reducing the cost of the products and would make them more competitive. For example, the amounts of dry ethanol used to produce silane (consumption coefficient of ethanol per unit of silane produced is 18-20) have a negative impact on the overall cost of the product. Another problem is a strict requirement to the allowed content of water in the alcohol (less than 500 ppm). Moreover, industrial alcohol contains up to 0.1% of water. Before it is used in the reaction with silicon, alcohol should go through an additional drying process where dehydrating agents, such as benzene (benzol), alkali metals, etc., are used. This additional dehydration step promotes contamination of alcohol and thus contamination of final products and by-products with impurities, and it increases the cost of alcohol.
Multistage distillation of the products of silicon-ethanol reaction (di-, tri-, tetraethoxysilane, unreacted alcohol etc.), in order to extract triethoxysilane, is energy consuming and, therefore, not efficient. Using alkali and alkali-earth metals as catalysts, for the simultaneous oxidation and reduction reaction, is also not efficient. Due to some secondary reactions, a layer of silicon forms on the surface of catalysts and deactivates them. Replacement of a catalyst during the reaction is a very dangerous operation. It also introduces a large amount of impurities into the process. Increasing temperature up to a point when it solves the problem of catalyst deactivation significantly increases the content of ethoxysilane vapors in the silane produced.
Another problem that has not been addressed in the precursor method is recycling of liquid secondary products, of the simultaneous oxidation and reduction reaction, which results in 25-30 times more liquid product than silane. This significantly increases consumption of raw material. Silane purification by low temperature condensation method is also not efficient because of the dispersed liquid phase that may form if the gas is overcooled. Purification of silane by impurity adsorption, when it contains more than one volume percent of vapors of organoelemental and silicon compounds, requires large quantities and frequent replacement of adsorbents.
It is therefor an object of the present invention to provide, in a method of high purity silane preparation, an increase in the output and purity of silane produced and, at the same time, reduce materials and energy consumption.
It is a further object of the present invention to increase the purity of produced silane through: (1) separation of unreacted dry alcohol from the products of silicon-alcohol reaction, at or near room temperature, and reusing alcohol in the process; (2) reusing the active part of a catalyst in simultaneous oxidation and reduction reaction of alcoxysilanes; (3) preliminary absorption purification of silane with cooled absorber followed by adsorption cleaning; and (4) return of dehydrated alcohol, resulting from complete hydrolysis of tetra-alcoxysilane subsequently purified, to react with silicon.
It is yet another object of the present invention to provide a method of high purity silane preparation which allows reduction of raw material consumption through: (1) return of alcohol, from all stages, to the beginning of the process; (2) reuse of the active catalyst portions; and (3) increase in the output of silane by reducing the amount of secondary products formed.
It is yet another object of the present invention to provide a method of high purity silane preparation which provides lower energy consumption through: (1) extracting unreacted alcohol from products of the reaction with silicon; (2) conducting simultaneous oxidation and reduction reaction in the presence of a catalyst in the continuous regime at or near room temperature; and (3) introducing the step of preliminary silane purification by absorption.
It is a further object of the present invention to provide a method of high purity silane preparation that is without wastes and it is ecologically safe.
These and other objects of the present invention will become apparent to one skilled in the art. All the foregoing objects provide a process which is economically effective and one which reduces the cost of silane. Polysilicon made from such silane is of sufficient quality that it is suitable for a whole range of semiconductor devices, including photovoltaic solar cells. Thus, the combination of the proposed processes and optimized regimes for high purity silane preparation provides the solution to the aforementioned problems.